Abstract

Bacterial abundances and production, and the size distribution of oxygen metabolism and
chlorophyll a concentration were followed through two seasonal cycles in the Menai Strait
(North Wales, U. K. ) and during austral summer in the Southern Ocean. In the Menai Strait,
spring blooms were characterised by a diatom to Phaeocystis succession. In both the Menai
Strait and the Southern Ocean, meso- and microphytoplankton dominated phytoplankton
production and biomass during diatom blooms. Nanophytoplankton predominated when
production and biomass were low, i. e. during the summer in the Menai Strait, in waters near
the Polar Front, and in some samples from the Weddell Sea.
In both ecosystems substantial respiration resided in the bacterial (< 0.8 gm) size-fraction.
Consequently during the Menai Strait temporal study, phasing of respiration in relation to
photosynthesis was strongly influenced by bacterial metabolism and abundance changes.
The respiration maximum occurred 1-2 weeks after the Phaeocystis abundance maximum.
An explanation for this temporal lag was sought by considering the time scales of flow of
organic material between the phytoplankton and the bacteria. The observations were
consistent with routes via a slowly cycling pool, such as polymeric organic material. This
pool would function as a reservoir and result in microheterotrophic respiration persisting
after the decline of photosynthesis, causing a positive to negative temporal sequence in net
community production.
There was no evidence for differences in any measure of microbial biomass between the
Southern Ocean and the Menai Strait. General relationships could be derived for both
ecosystems: (a) the biomass quotient (< 20 µm phytoplankton / unfractionated
phytoplankton) generally increased sharply as unfractionated phytoplankton biomass
decreased, (b) bacterial biomass generally increased as phytoplankton biomass increased,
(c) the biomass quotient of bacteria to unfractionated phytoplankton increased sharply as
unfractionated phytoplankton biomass decreased.
Different relationships were derived for the oxygen fluxes in terms of phytoplankton
biomass for the Southern Ocean and Menai Strait observations. In these relationships, the
oxygen fluxes were generally relatively (relative to the explanatory variable: phytoplankton
biomass) higher in the Menai Strait. In contrast, a single relationship for DCR in terms of
GCP was fitted for both data sets. This difference is consistent with a temperature effect on
the oxygen fluxes, with GCP and DCR similarly suppressed at lower temperatures.